Short answer: NO!
Longer answer: H**L NO!
By the time you get the voltage regulator adjusted to keep the thing charged, the voltage will be about 9.5 volts, and every 6-volt part on the truck will be overloaded. it will burn points, burn out light bulbs, and go through generators and regulators like popcorn at a double-feature movie! If you're determined to get lots of cranking voltage, get two 6-volt batteries and a series-parallel switch, and put 12 volts to the starter only, none of the rest of the electrical system. It's expensive and complicated to wire, but it doesn't involve overstressing the whole electrical system, just the starter.
Jerry
